1. Field of the Invention
The present invention relates to an electron beam source device with a filament for emitting hot electrons, such as an ion source for a mass spectrometer or a gas chromatography mass spectrometer in the analysis field, atom force field, vacuum field, and semiconductor field, etc.
2. Description of Related Art
In the following, an ion source used in a gas chromatography mass spectrometer (e.g., refer to Reference 1) is used as an example, and the construction and operation of a conventional electron beam source device are illustrated with reference to FIG. 4. FIG. 4 is a sectional view of the main construction of an ion source, and also illustrates the peripheral circuits relevant to the operation of the ion source. The ion source exhausts the air through a common vacuum exhaust device (not shown), so as to be operated in a vacuum environment. A filament F is heated by the electric power supplied by a filament power supply 1, so as to emit hot electrons. The emitted hot electron stream (hereinafter referred as an emission current) is accelerated to move towards an ionization chamber 2 upon being applied with an electron accelerating voltage of about 100 V by an electron acceleration power supply 3 disposed between the filament F and the box-shaped ionization chamber 2. The emission current is incident into the ionization chamber 2 from one of the openings disposed on the ionization chamber 2, and exits the ionization chamber 2 from another opening disposed on the opposite side of the above opening, so as to be collected in an electron collector 4. In order to effectively collect the emission current to the electron collector 4, in most cases, a magnetic field (not shown), for example, about several mT in magnitude, is applied in the traveling direction of the emission current in the ion source. If secondary electrons generated when the hot electrons crash the electron collector 4 flow into other electrodes such as the ionization chamber 2, etc., the external emission current will reduce the corresponding emitting volume of the secondary electrons; thus, an error occurs between the emission current and a standard emission current. However, as a voltage of several tens of volts is generally applied between the ionization chamber 2 and the electron collector 4 by a collector power supply 5, the generated secondary electrons are influenced by a repulsive force of the ionization chamber 2 that is at a ground potential, i.e., a relatively negative potential. The generated secondary electrons are again attracted by the electron collector 4, and the influence of the secondary electron is thereby obviated.
While being measured and displayed through an emission current meter 6, the value of the emission current collected by the electron collector 4 is further provided to an emission controller 7. An output of the emission controller 7 is fed back to the filament power supply 1, so as to increase/decrease the output of the filament power supply 1. In other words, when the emission current incident into the electron collector 4 is larger than a specified value, the emission controller 7 controls the output of the filament power supply 1 to reduce the electric power for heating the filament F. On the other hand, when the emission current incident into the electron collector 4 is less than the specified value, the emission controller 7 controls the output of the filament power supply 1 to increase the electric power for heating the filament F.
At one side of the ionization chamber 2, samples to be measured are introduced therein from a sample inlet opening 8. The samples to be measured are ionized in the ionization chamber 2 by the emission current, and they are guided out of the ionization chamber 2 from an ion outlet opening 9 in a direction at the back of the figure, so as to be incident into a mass selection portion (not shown) to perform the mass analysis. One portion of the filament F sublimates due to being heated; thus, the filament F needs to be replaced repeatedly. Generally, the period for replacing the filament F is determined according to an accumulated light-on time of the filament F used under an assumed standard emission current.
[Reference 1] JP Patent Laid Open H11-86778
The construction and operation of the conventional electron beam source device are described in abovementioned reference; however, with the construction electron beam source device described in the reference, as the period for replacing the filament merely determined by the accumulated light-on time, the filaments that still can be used may also be replaced. Thus, resources are wasted the replacement cost is increased. Accordingly, as mentioned above, the conventional replacement period is determined according to the accumulated light-on time of the filament used under the assumed standard emission current. However, under most cases, the emission current varies depending upon the different using conditions, and the accumulated light-on time is not exactly corresponding to the consumption of the filament; thus, the accuracy of the prediction is relatively low. For example, when being used under a smaller emission current, the filament that is still useful will be replaced and wasted. Thus, as described above, the resources are wasted, which is just the reason that causes the increase of the replacement cost. On the contrary, when being used under a large emission current, the filament will be disconnected in a shorter accumulated light-on time. Thus, the sudden disconnection interrupts the operations such as analysis, and disturbs the procedures.
Moreover, in the conventional construction, the quality difference among the filaments cannot be taken into consideration due to the problem in quality management. For example, even if a particular filament with a shorter life span than the standard filament is going to be broken, this situation cannot be predicted in the conventional device from the external. As a result, the necessary measures can be taken only after the disconnection occurs, thus causing an interruption of the operations such as analysis and causing the delay of the procedure. In addition, due to a similar quality management problem, for example, even if variation or instability of the analysis property occurs due to the deformation or short circuit of a particular filament in use, also similar to the above circumstance of disconnection, such a situation cannot be predicted beforehand from the external, so the property variation or instability is determined after the data is analyzed, and whether the filament has any problem or whether the filament is deformed or not can only be determined through the process of examining and repairing of the faulted device. The determination process requires certain duration of time. Therefore, the time spent on determining the problem with the time spent on repairing and reanalysis after the determination of the problem delays the procedure. The present invention is directed to providing a solution for such a problem.